Ensure Environment Dashboard works with external deployments

Sooner or later, we'll have external deployments, which is a deployment object without pipelines/jobs associations as the deployment performed outside of the GitLab CI/CD (More context in !17620 (merged)).

We need to ensure that Environment Dashboard works with this case. For example, external deployments cannot show pipelines/jobs statuses. It could break the UI/UX proposal in #3713 (closed) which created based on an assumption that all deployments are internal.

/cc @jagood @ogolowinski @rverissimo @afontaine

Follow-up from "Fill in Pipeline Footer in Environment Dashboard"

The following discussion from !17645 (merged) should be addressed:

  • @dosuken123 started a discussion: (+7 comments)

    What happens if a deployable (pipelines/jobs) doesn't exist in the deployment? How does the UI behave?

Edited by Rayana Verissimo